Method for the production of micro/nanofluidic devices for flow control and resulting device

ABSTRACT

The present invention refers to a procedure for the manufacture of micro-nanofluidic devices for flow control such as microvalves, micropumps and flow regulators, using a photodefinable polymer and an elastomer as structural materials and to the micro-nanofluidic devices for flow control obtained by said procedure.

FIELD OF THE INVENTION

The present invention refers to the field of micro-nanofluidic controldevices; specifically, it refers to membrane microvalves, pneumaticmicropumps and flow regulators, manufactured with polymeric materials.

STATE OF THE ART

The first valve with micrometric dimensions was introduced in 1979 byFerry S. C. [1], and even though in the last 20 years there has beengreat progress there are still many improvements to be made as regardsthe characteristics of the microvalves [2].

Recently, a great effort has been made in the development of mechanicalmicrovalves to try to attain a short response time (ms), with low losses(□1/min.) when high pressures are used (bar). Most devices developed sofar have been produced using MEMS (Micro-electro-mechanical Systems)technology.

As regards the actuation methods, the most important ones are magnetic,electrical, piezoelectric and thermal ones. However, none of them canguarantee on its own the good operation of the microvalve, since theyultimately depend on the final application it is destined for.

The magnetic actuation is usually carried out through the use ofsolenoids to move membranes or pistons [1, 3-6], since they can generategreat forces and fast responses. However, for miniaturized structures,the electrostatic actuation is more attractive. Nevertheless, in orderto obtain great forces and displacements it is necessary to use highvoltages [7-10], which is not always possible or convenient.

The piezoelectric actuation enables to obtain very high forces but verysmall displacement even for high voltage levels [11-14].

The thermal actuation can also produce elevated forces, but itsrelatively slow response and the heat dissipation produced may adviseagainst its use in many cases [15-18].

Finally, the use of an external actuation method can be useful to complywith these requirements, although it can comprise the miniaturization ofthe device (pressure through an external pin) or its portability(pneumatic pressure) [19-25].

Through the use of the same techniques detailed in the previousparagraph, it is possible to carry out the manufacture of the micropumpsbased on reciprocal displacement. This kind of pumps basically consistof the manufacture of a pumping chamber where the volume to be displacedis located, and two microvalves. Through a cyclic actuation of thevalves and the pumping chamber, it is possible to attain a net flow inthe required direction [26-30].

The most promising applications for the microvalves and micropumps arebiotechnology and miniaturized chemical laboratories. The turning on andoff of the microfluidic channels, or the sealing of biomolecules andchemical agents without losses, even when high pressures are applied,are critical properties to guarantee the good functioning of thebiochemical chip. The loss of biological or chemical agents, and theirmigration to different areas of the chip could lead to its contaminationand have a negative influence on the reactions to be produced. Thesealing of the reaction chambers is also essential to prevent theevaporation of agents and the generation of air bubbles if hightemperature is used.

Some polymeric materials, as they are biocompatible, have an importantadvantage. Besides, their great elasticity turns them into excellentcandidates for membranes which can displace along great distances (up tohundreds of micrometers). Therefore, they are materials which areoptimal to manufacture microvalves, micropumps and flow regulators basedon active designs by displacement.

As regards the polymeric microvalves, although there have been presentedworks in which the actuation is carried out by different principles, themost frequently used one is the external pneumatic actuation (presenceof air pressure or vacuum). This occurs since this kind of actuation hasadvantages to obtain valves without losses under high pressure, althoughthe miniaturization may be difficult. In case of micropumps, it isnecessary to highlight that the only micropump which is commerciallyextended is made through plastic microinjection (thinXXS GMBH, Germany).

In great part of the aforementioned works based on polymers, thematerial used as membrane is an elastomer, being PDMS the mostfrequently used one. Its low Young's modulus makes it the ideal materialto be actuated. Besides, its elasticity can be varied according to thecuring agent percentage added to the final mixture [31]. However, incases where the PDMS is also used as structural material, the final chipends up being mechanically unstable (too flexible). In order to preventthis, substrates such as glass or silicon are used, which considerablyincrease the final price of the chip. Moreover, the use of PDMS asstructural material of the channels makes these suffer deformations dueto the pressure of the fluid going through them, which affects thefluidic control of the device [32].

Therefore, there is a need to find a micro-nanofluidic control devicewhich work is not limited by manufacturing materials.

DESCRIPTION OF THE INVENTION

The present invention provides a procedure for the manufacture ofmicrovalves, micropumps and polymeric flow regulators using aphotodefinable polymer and an elastomer as structural materials so thatthe final device is not deformed by the fluid pressure, also having amobile membrane with a low Young's modulus, creating a broad range ofpossibilities for its actuation.

In a first aspect, the present invention refers to a procedure for themanufacture of micronanofluidic devices for flow control, characterizedin that it comprises the following stages:

-   -   a) Deposition and definition of a photodefinable polymeric        layer, on an independent substrate surface (wafer 1),    -   b) Deposition and definition of a photodefinable polymeric        layer, on an independent substrate surface (wafer 2) covered by        a non-stick material,    -   c) Sealing of the wafer 1 and the wafer 2, facing said wafers        the face containing the photodefinable polymeric material,    -   d) Removal of the wafer 2 covered by non-stick material,    -   e) Deposition of a metallic layer on the polymeric layer        resulting from stage d),    -   f) Deposition and definition of a photodefinable polymeric layer        on an independent substrate surface (wafer 3) covered by a        non-stick material,    -   g) Sealing of the wafer 1 and the wafer 3, facing said wafers        the face containing the photodefinable polymeric material,    -   h) Removal of the wafer 3 covered by non-stick material,    -   i) Spinning and curing of an elastomeric layer on an independent        substrate of non-stick material with the elastomer (wafer 4),    -   j) Sealing of the wafer 1 and the wafer 4 facing the        photodefinable polymeric material and the elastomeric layer,    -   k) Removal of the wafer 4    -   l) Deposition and definition of a crystalline structure material        on the elastomer face which is bare at stage K),    -   m) Deposition and definition of a photodefinable polymeric layer        on an independent substrate surface (wafer 5) covered by a        non-stick material,    -   n) Sealing of wafer 1 and wafer 5, facing said wafers the face        containing the elastomer and the photodefinable polymeric        material respectively,    -   o) Removal of the wafer 5 covered by non-stick material,    -   p) Deposition and definition of a photodefinable polymeric layer        on an independent substrate surface (wafer 6) covered by a        non-stick material,    -   q) Sealing of wafer 1 and wafer 6, facing said wafers the face        containing the photodefinable polymeric material,    -   r) Removal of the wafer 6 covered by non-stick material.

In a particular aspect, the procedure of the present invention comprisesa stage previous to stage a) consisting on the deposition and definitionof a metallic layer on the surface of the independent substrate (wafer1). Particularly, the metallizing of the present invention is carriedout by techniques known by an expert in the art, such as cathodesputtering, evaporation or deposition.

In another particular aspect of the present invention, the crystallinematerial deposited at stage k) is a metal.

In another particular aspect of the present invention, the crystallinematerial deposited at stage k) is a piezoelectric material.

In another particular aspect of the present invention, the sealing atstages c), g), j), n) and q) is carried out through the application ofpressure and temperature.

In another particular aspect of the present invention, the sealing atstage n) comprises a previous treatment with oxygen plasma.

In another particular aspect of the present invention, thephotodefinable polymeric layer deposited at stages a), b), f), m) and p)consists of the photoresin SU-8.

In another particular aspect of the present invention, at stages a), b),f), m) or p) more than one photodefinable polymeric layer is depositedand defined.

In another particular aspect of the present invention, the elastomerlayer is PDMS.

In another particular aspect of the present invention, themicro-nanofluidic device for flow control is a micropump.

In another particular aspect of the present invention, themicro-nanofluidic device for flow control is a microvalve.

In another particular aspect of the present invention, themicro-nanofluidic device for flow control is a flow regulator.

In a second aspect, the present invention refers to a micro-nanofluidicdevice for flow control obtained through the previously describedprocedure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the contact angle measured for different plasmaconfigurations.

FIG. 2 shows SEM images of the interface between SU-8 (1) (an example ofphotodefinable polymer) and PDMS (2) (an example of elastomer).

FIG. 3 shows the design proposed as valve and flow regulator, a) opencross section, b) closed cross section, c) open upper view and d) closedupper view, as it can be seen, two channels absorbed in polymeric layerswhich are connected through two open inlets located in a common chamber.In said chamber the elastomeric membrane (3) is located through thepasting process described before at a height determined by a new polymerlayer.

FIG. 4 shows the cross section of a microfluidic device (microvalve)with different distances between the elastomeric membrane (3) and thefloor.

FIG. 5 shows the cross section of a microfluidic pump in a cyclicalseries of actuations a), b) and c), where it can be seen a centralchamber (5) and two valves (4) at the ends.

FIG. 6 shows the general scheme of the procedure for the manufacture ofmicrovalves (4), micropumps (6) and flow regulator (7).

FIG. 7 shows a flow regulator comprising an extra electrode (8) in theupper part.

DETAILED DESCRIPTION OF THE INVENTION

The procedure of the present invention is based on the use ofphotolithography and sealing techniques at temperatures under 100° C. ofthermosetting polymeric materials such as SU-8 (U.S. Pat. No. 4,882,245,Nov. 21, 1989), which allow the manufacture of very high resolutionmicrofluidic structures (between 500 nanometers at 500 micron width andbetween 1 and 200 micron height) on rigid substrates with good bonding(for the final substrate of the devices), or on rigid substrates onwhich a plastic material on which SU-8 has slow bonding has beendeposited, pasted or laminated, in a reversible manner, such aspolyimide (kapton), PET or Mylar. These polymeric materials of lowbonding to SU-8 enable us to release the chips from the substrate onwhich new layers of SU-8 have been added. Besides, in the presentinvention there have been developed manufacturing processes to integrateelastomeric membranes (PDMS) and methods for metallizing the differentlayers.

The manufacturing process of the invention started with the deposit ofSU-8 photoresin through the spinning on a transparent rigid wafer. Itcan be a glass, or polymeric (PMMA, COC, etc.) wafer, but in thisconcrete case a PMMA wafer was used. The thickness of the layerdeposited depends on the speed and the spinning time, and on theformulation of the SU-8 (quantity of solvent inserted on the polymer),being possible the spinning of layers of 200 nanometer thickness up to500 micron thickness. In this case layers of 20 and 40 micron thicknesshave been used. If it was desired to obtain thicker layers, the spinningprocess was repeated until the required thickness was obtained. Next,these layers were submitted to a thermal process (soft bake) at atemperature between 65 and 100° C., during 7 to 30 minutes, according tothe temperature and thickness used. In this process the rest of thesolvent was evaporated and the resin deposited was standardized. Oncethis thermal process has ended, the polymer deposited wasmicrostructured using standard photolithography techniques. With thehelp of a mask placed on top of the wafer, the areas to be kept areexposed to ultraviolet light. The dose used was 140 mJ/cm². Later, thelayers were submitted to a thermal process for the partial curing of thephotoresin. This process occurred at a temperature between 65 and 100°C., during 1 to 10 minutes, according to the temperature and thicknessused. Once room temperature was reached, a chemical development of thenon-exposed areas was performed, defining in the photoresin the formsdrawn on the mask used for the ultraviolet lighting process. Thisdevelopment was carried out with the standard developer of the SU-8photoresin (propylene glycol monomethyl ether acetate, PGMEA).

The SU-8 layer definition process was also carried out in PMMA wafers onwhich a material with low bonding with SU-8 has been deposited in areversible manner. These layers have the purpose of being added to thefinal substrate by means of a sealing process. This sealing has alreadybeen detailed in literature, both when silicon and glass wafers [33-34]are used and when PMMA wafers are used [35]. However, in both cases theyare limited to SU-8 layers defined with thermal treatments which are ataround 90° C., using a pressure of 3.25 bar when silicon or Pyrex areused as substrate and 3.75 bar when PMMA is used. In the presentinvention it was also carried out pasting of the SU-8 layers and theirdefinition process was carried out through thermal processes which rangefrom 65° C. to 120° C. The pasting can be performed at temperatures aslow as 75° C., and as high as 120° C. The result in all cases is apercentage of pasting area of 100%.

Once the sealing process has ended, the devices are submerged in anisopropanol bath and submitted to ultrasound for 1 minute. This bath hastwo functions, the releasing of the Kapton, PEB or Mylar layers of therigid substrates and the cleaning of the devices.

During the manufacture of the devices described in the present inventionit is required the sealing of SU-8 layers with flexible membranes basedon elastomers. In this particular case, PDMS has been used, which wasspinned in PMMA wafers until the desired thickness was obtained and leftfor curing (24 hours at room temperature). Later, both the wafer withSU-8 and the one containing PDMS are treated with oxygen plasma to boostits bonding. The specific conditions used where 2 minutes at 18 W,although more combinations of power and time are also valid (see FIG.1). The sealing is carried out under the same conditions under which thepasting of SU-8 with SU-8 is carried out [36]. Finally, the PMMsubstrate is removed manually and cleanly thanks to the low bondingbetween PDMS and PMMA.

The pasting of the photodefinable polymer and elastomer to which thepresent invention refers is based on the use of previous treatment withoxygen plasma in both materials. They are later put in contact throughpressure and temperature. During the plasma treatment both the elastomerand the photodefinable polymer undergo a superficial molecularbreakdown, increasing its possibilities for a possible union betweenthem. Both materials experience an increase of hydrophilicity accordingto the success of the plasma treatment. Therefore, the effect of thissuperficial treatment can be measured indirectly through the measurementof the angle of contact of one drop of water deposited on it. In thisway, it is possible to calculate the optimum parameters for the plasmatreatment to carry out the sealing [33].

Once both the photodefinable polymer and the elastomer have been treatedwith oxygen plasma, both materials are put in contact. At this point, itcan be observed that the first bonding starts to occur between the twopolymers, although they can still be easily separated probably due tothe reduced contact surface. However, when they are submitted to apressure of 325 KPa and to a temperature above 65° C. and under 120° C.,the pasting becomes irreversible. While applying pressure considerablyincreases the contact surface between the two polymers, the thermaltreatment acts as catalyst for the polymerization of the polymer withthe elastomer. The pasting strength was tested trying to separate bothpolymers applying air pressure. Both, when the polymer is pasted to theelastomer excluding the oxygen plasma, and when the application ofpressure and/or temperature is eliminated, both materials are separatedeasily. However, when the photodefinable polymer and the elastomer aresubmitted to the oxygen plasma and to the action of pressure andtemperature, their separation through air pressure was impossible. Bycontrast, it can be observed that the elastomeric membrane breaksinstead of removing itself from the polymer (2.5 bar when the elastomerconsists of a PDMS layer with a thickness of about 80 μm and a mixtureof 1:10 with respect to the curing agent), thus proving that the pastingis irreversible. FIG. 2 shows SEM images of the interface between SU-8(an example of photodefinable polymer) and PDMS (an example ofelastomer). It is therefore proved the integration of an elastomer and aphotodefinable polymer for the manufacture of microfluidic devices.

In case of using the design of FIG. 3 as a valve, the elastomericmembrane is actuated until it is crashed against the floor. As a result,the two holes that allow the flow from one channel to the next areblocked and the flow through the channels stops. However, when theactuation on the elastomer is released it goes back to its originalposition, allowing the flow to go through again. It is therefore anormally open valve. The design in FIG. 3 can be also used for themanufacture of flow regulators.

In this case the elastomeric membrane is actuated so that its distanceis reduced with respect to the floor without blocking the inlets of thetwo channels, as shown in FIG. 4. In this way, the flow that goesthrough the channel is reduced due to the increase of fluidic resistancegenerated by the displacement of the elastomeric membrane. Thedimensions of the device as regards channel size, connecting holes sizeand distance between them, determine the range of flow that it cancontrol.

Finally, this technology is also proposed for the manufacture ofmicrofluidic pumps based on reciprocal movement. As shown in FIG. 5, thepump consists of a chamber containing the liquid to be displaced in eachcycle. The displacement of said liquid is based on the same principlepreviously described, that is, the actuation of an elastomeric membrane.On both sides of the chamber, two valves are used to direct the flow ofthe liquid. A possible way is the use of a cyclical series ofactuations, from which a net flow is obtained in the required direction(see FIG. 5). However, the use of active valves can be avoided andpassive valves can be employed (check valve). Mobile structures made ofphotodefinable polymer [34-35] can be used to favour the flow in onedirection, being necessary the only actuation of the elastomericmembrane.

In case of some kinds of actuation (electric, thermal, magnetic, etc.),the procedure of the invention requires a stage previous to the depositand definition of the photodefinable polymeric layer (SU-8), of metallicmaterial deposition. These depositions can be by evaporation techniquesor by cathode sputtering, being possible to use deposition techniques ifthicknesses of around dozens of microns are required such as in the caseof magnetic field actuation. The definition of said layers can be easilymade through a physical mask (“shadow mask”), which is placed on top ofthe wafer where it is desired to deposit the metallic material to allowthe deposition only in the desired areas. For said metallizations tohave an optimal bonding, the surfaces are treated with oxygen plasmausing the data of FIG. 1.

The steps of the deposition of photodefinable polymer layers can berepeated more than once to obtain greater thicknesses.

FIG. 6 shows the manufacturing scheme as described in the description ofthe invention section. Although the last two polymer layers and theirsealing are only indispensable for pneumatic actuation, it is alsoadvisable to use it in case of actuation through magnetic and electricfield as such, as shown in FIG. 7. In this way, there is an increase ofthe volume displaced by the pump and the maximum actuation voltageremains constant and independent of the pressure at which the fluid issubmitted. Said pressure causes the bulking of the elastomeric membraneand if there is no stop, the voltage required for the actuation of themembrane can be excessively high.

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1. Procedure for the manufacture of microfluidic devices for flowcontrol using a photodefinable polymer and an elastomer as structuralmaterials wherein it comprises the stage of pasting the photodefinablepolymer and the elastomer based on the use of previous treatment withoxygen plasma in both materials and, once both the photodefinablepolymer and the elastomer have been treated with oxygen plasma, bothmaterials are put in contact and submitted to a pressure of 325 KPa andto a temperature above 65° C. and under 120° C., so that the pastingbecomes irreversible and the final device is not deformed by the fluidpressure.
 2. Procedure for the manufacture of microfluidic devices forflow control according to claim 1, wherein the photodefinable polymerpasted consist of photoresin SU-8.
 3. Procedure for the manufacture ofmicrofluidic devices for flow control according to claim 1, wherein theelastomer pasted is PDMS.
 4. Procedure for the manufacture ofmicrofluidic devices for flow control according to claim 1, wherein themicrofluidic devices are micropumps, microvalves and/or flow regulators.5. Microfluidic device for flow control obtained by the proceduredescribed according to claim 1.